fix(pose): clear iphone hands on stream drop

iphone_usb_source.py: after stream ends, clear
persons_hands_iphone, persons_hands_chirality, and
persons_hands (when write_hands=True) under state.lock()
to prevent renderer display freeze.

test_iphone_usb_reconnect.py: add
test_hands_cleared_on_stream_drop to verify lists are
empty after simulated USB disconnect.

data_only_viz/iphone_usb_source.py

@@ -239,6 +239,14 @@ class IphoneUSBSource:
             # USB hiccup ends iter_frames; we re-establish :7000).
             with self._lock:
                 self._frame = None
+            # Clear stale hand state so the renderer doesn't freeze on the
+            # last known hands after a stream drop.
+            if self.state is not None:
+                with self.state.lock():
+                    self.state.persons_hands_iphone = []
+                    self.state.persons_hands_chirality = []
+                    if self._write_hands:
+                        self.state.persons_hands = []
             self._opened = False
             sock = None                           # forces the reconnect at the loop top
             if not self._stop.is_set():

data_only_viz/tests/test_iphone_usb_reconnect.py

@@ -4,6 +4,7 @@ race (app not ready when data_only_viz starts), not only a mid-session drop."""
 import time
 
 import data_only_viz.iphone_usb_source as m
+from data_only_viz.state import PoseKp, State
 
 
 def test_start_keeps_retrying_when_no_device(monkeypatch):
@@ -23,3 +24,42 @@ def test_start_keeps_retrying_when_no_device(monkeypatch):
             "start() should keep retrying; connect_device called %d time(s)" % calls["n"])
     finally:
         src.release()
+
+
+def test_hands_cleared_on_stream_drop(monkeypatch):
+    """After a USB stream drop, persons_hands* lists must be emptied so the
+    renderer does not freeze on the last known hand positions."""
+
+    class FakeSock:
+        def close(self): pass
+        def shutdown(self, *a): pass
+
+    connect_calls = [0]
+
+    def fake_connect():
+        connect_calls[0] += 1
+        if connect_calls[0] == 1:
+            return FakeSock()   # initial connection succeeds
+        return None             # reconnect fails; release() will interrupt the wait
+
+    def fake_iter_frames(sock):
+        raise OSError("stream dropped")  # simulate immediate disconnect
+
+    state = State()
+    dummy_hand = [PoseKp(x=0.5, y=0.5, z=0.0, c=1.0)] * 21
+    with state.lock():
+        state.persons_hands = [dummy_hand]
+        state.persons_hands_iphone = [dummy_hand]
+        state.persons_hands_chirality = [1]
+
+    monkeypatch.setattr(m, "connect_device", fake_connect)
+    monkeypatch.setattr(m, "iter_frames", fake_iter_frames)
+
+    src = m.IphoneUSBSource(state=state, write_hands=True)
+    src.start()
+    src.release()   # joins thread; ensures clearing happened before we check
+
+    with state.lock():
+        assert state.persons_hands == [], "persons_hands not cleared on drop"
+        assert state.persons_hands_iphone == [], "persons_hands_iphone not cleared on drop"
+        assert state.persons_hands_chirality == [], "persons_hands_chirality not cleared on drop"